A core assembly is described in non-prepublished German Patent Application No. DE 10 2004 008986.
When a core assembly is mounted in an ignition coil, the core assembly is positioned in the ignition coil housing during premounting. The premounted ignition coil is then cast in a casting chamber according to a vacuum casting process, using a casting resin made of epoxy resin. Casting takes place in a vacuum to be able to fill all cavities of the ignition coil with the epoxy resin or casting resin and impregnate the windings. Because the core stack is surrounded by a sheath, usually in the form of a heat-shrinkable sleeve, as well as a damping element and a cover on the side of the core assembly diametrically opposed to the damping element, a pressure gradient is produced between the interior of the core assembly and its surroundings upon evacuation of the casting chamber. However, the pressure gradient decreases only very slowly, so that air exits the core assembly even during the ignition coil casting process carried out in a vacuum, which may cause bubbles to form in the cured casting resin or even at its surface. The formation of bubbles may impair the function of the bar-type ignition coil over the course of its life. As a result, an attempt is made to remove the air present in the coil as early as a prevacuum step, which takes place prior to casting. German Patent Application No. DE 10 2004 008986 also describes an air passage in the form of at least one slot provided in the damping element to improve the discharge of air from the core area.
An object of the present invention is to improve the known core assembly through alternative air evacuation means to further minimize or completely prevent the entrapment of air bubbles during casting of the core assembly.